Corner filters have been used for many years, at one time these filters probably introduced more people to this hobby than any other filter. These filters' strong points are they are cheap and easy to maintain. So how do they work ? Corner filters are air driven (an air pump is needed) using an uplift tube. Water is drawn into a chamber containing filter floss for the mechanical filtration and activated carbon for the chemical filtration. These filters are effective (mainly for smaller tanks) and easy to use. Remember the chamber should be cleaned on a regular basis. Also the draw or pull (water flow) is low so debris will tend to build up in the substrate. This requires regular water changes using a good gravel siphon.
Sponge filters are also air driven and mainly for smaller tanks. However some models can be driven by a power head for increased water flow. Again these filters are effective and inexpensive. There is a trick to these filters, the sponge portion should be rinsed in dechlorinated water. Sponge filters start out as a mechanical filter trapping debris, however, after six weeks they establish enough bacteria by feeding on this waste to become quite effective. Just remember if you use a chlorinated water source or clean these filters too well this bacteria colony will be destroyed. Again these filters usually have low flow rates so debris will collect at the bottom of your tank requiring regular water changes using a good gravel siphon.
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